Journal of Electron Spectroscopy and Related Phenomena, 32 (1983) 153-162 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

He(I) AND He(I1) PHOTOELECTRON INVESTIGATION OF THE ELECTRONIC STRUCTURE OF TROPOLONE AND OF THE RELATED ANION LIGAND

GIOVANNI BRUNO, GABRIELE CENTINEO, ENRICO CILIBERTO and

IGNAZIO FRAGALA

Zstituto Dipartimentale di Chimica, Universitb di Catania, Viale A. Doria 8, 95125 Catania (Italy)

ABSTRACT

He(I) and He(I1) photoelectron spectra of gas-phase tropolone, 2-methoxytropone and the thallium(I) tropolonate complex are reported. Comparison with the PE spectra of related molecules as well as with semiempirical quantum-mechanical calculations are used in making assignments. Remarkable analogies are apparent with the spectrum of the parent molecule tropone. Effects due to intramolecular hydrogen bonding account for some details of the tropolone spectrum. Comparison of the PE spectra of neutral tropolone and of its Tl(1) complex leads to a reasonable description of the electronic structure of the tropolonate anion ligand.

INTRODUCTION

The tropolonate anion ligand forms stable complexes with nearly all metal ions. These complexes closely resemble the corresponding P-diketonate ligands. There are, however, significant differences, since the tropolonate ligand forms five-membered chelate rings in which the coordinated oxygen atoms exhibit a smaller intraligand distance than encountered in related 6-diketonate derivatives. This compact ligand shape, as well as its skeletal rigidity and the relatively smaller distance between the oxygen atoms, make this ligand particularly effective in forming highly coordinated structures [ 11. Whereas systematic investigations of P-diketonate systems [ 21 are available, there is a complete lack of PE data for tropolonates. In this paper we report PE spectra of tropolone, of 2-methoxytropone and of the Tl(1) tropolonate complex. Study of the Tl(1) complex [3--51 is particularly suited to elucidate the electronic structure of the bonded anion ligand. The question of the “aromaticity” [6] of tropolone and related molecules has produced many papers dealing with theoretical and experimental studies [ 71. The PE spectra of tropone and of several related derivatives have been

0363-2043/33/$03.00 0 1983 Elsevler Science Publishers B.V.

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measured and carefully assigned [ 81, but, as far as we know, the gas-phase PE spectrum of tropolone has not yet been published. By contrast, the X-ray PE spectrum of tropolone [9] in the solid state has been discussed in relation to the question of hydrogen bonding.

EXPERIMENTAL

Commercial tropolone (Fluka) was purified by sublimation in vacuum. 2Methoxytropone was prepared according to procedures proposed for the alkylation of phenols [ 10 1. The pale-yellow liquid was characterized satis- factorily by NMR [ 111 and UV spectroscopy [ 121. The Tl(1) tropolonate complex was prepared according to the literature [ 131.

The photoelectron spectra were obtained using a Perkin-Elmer PS18 photoelectron spectrometer, modified for He(I1) measurements by the inclusion of a hollow-cathode discharge lamp (Helectros). All spectra were calibrated by reference to the peaks due to a mixture of argon, xenon and nitrogen, and to He ls-1 self-ionization. Ionization-energy (1) data are collected in Table 1, together with related literature data and the results of semiempirical quantum-mechanical calculations (INDO/S and HAM/3) for neutral tropolone. The geometries of tropolone were taken from gas- phase electrondiffraction studies [14]. The structure adopted differs from that suggested by solid-state X-raydiffraction data [15] but agrees well with the results from NMR [ 121 and near-UV spectroscopic [16] studies. The geometries of the tropolonate anion were taken from the literature [17, 181. They are discussed in the text in relation to the results of CNDO calculations for Na( I) tropolonate .

RESULTS AND DISCUSSION

The low-ionization-energy region (up to 12eV) in the spectrum of tropolone contains four bands (labeled l-4 in Fig. 1). Band 4 is not dis- cussed, because the assignment remains uncertain. Vibrational structures are associated with bands 1 and 2 (Fig. l(b)). The INDO/S and HAM/3 cal- culations (Table 1) for tropolone (C, symmetry) suggest that the three uppermost molecular orbitals (MO’s) are the n(5u”), having considerable contributions from the exocyclic oxygen orbitals (30%), the n (l&z’) “lone pair”, localized mainly on the carbonyl oxygen atom, and, finally, a further x(4e”) MO localized mainly on the ring carbon atoms (85%). Obviously, the l&z’ MO is significantly admixed with other orbitals of cr symmetry centered on adjacent atoms. Ionization of the hydroxylic lone pair is predicted at higher I, in accordance with other literature data [ 191. This orbital sequence is consistent with the assignment already proposed [8a] for the lower-l

166

9 10 11 eV

Fig. 1. (a) He(I) and He(I1) PE spectra of tropolone; (b) low-energy region of He(I) PE spectrum on expanded scale.

b, 6-1 CO a&l

I i I I I I I 0

(1 cna 6 I \ -

I I I 0

8 9 10 11 l V

Fig. 2. Correlation diagram for tropolone and related molecules.

region of the PE spectrum of the parent molecule tropone (n(3bI ) < n(lObz)<r(laz) in C 2v symmetry), thus suggesting that the PE spectrum of tropolone can be interpreted in terms of a hydroxy-substituted tropone. A correlation diagram for the uppermost filled orbit& of tropolone and of related molecules is pictured in Fig. 2. The spectrum of tropolone resembles

167

He II

Hel

-i-r-- 12 ev

Fig. 3. He(I) and He(I1) PE spectra of methoxytropone: low-energy regions.

that of the parent molecule tropone, although the ,-I ionization (labeled 2 in Fig. 1) appears considerably shifted towards higher I. Different intensity ratios among various bands are also observed. Furthermore, comparison (Table 1) of the I data for the two molecules indicates that all the bands in the spectrum of tropolone are shifted to lower I with the sole exception of band 2. The general trend of I under 2-hydroxy substitution is qualitatively similar to that observed in the case of a-methyl substitution (see I data for 2-methyltropone [8a] in Table 1). This observation again applies to the first and third bands.

A complete analogy, however, becomes apparent when comparing the spectra of 2-methyltropone and 2-methoxytropone. Both spectra show three bands in the low-1 region (Table l)*. The spectrum of 2-methoxytropone (Fig. 3) can be assigned on a purely experimental basis by simple analogy to the assignment already proposed for the spectra of 2-methyltropone and tropone (Table 1). The 2-methoxy and a-methyl substituents induce shifts to lower I of all the bands below 11 eV (Table 1). However, the effect is more pronounced in the case of the 2-methoxy compound, and, as expected, involves mainly the bands related to ionization of nb;’ and ~cz;’ (bands 1 and 3, respectively). Furthermore, it is worth noting that the broadening of

* This trend is alSo confiied by PE data for some bridged tropones kindly supplied by Prof. E. Heilbronner.

158

the n,$ bands is consistent with extensive mixing of the carbonyl “lone pair” with other suitable MO’s [8a] .

We are now in a position to assign the spectrum of tropolone. It must be expected to be similar to that of 2-methoxytropone, since hydroxy or methoxy substituents have similar inductive effects. However, in the case of tropolone an intramolecular hydrogen bond can be formed within the a-dicarbonyl fragment. Brown [20] has suggested that hydrogen bonding increases the n<Lo ionization energy. This trend is very well reproduced if the broad band 2 at 9.80eV in the spectrum of tropolone is assigned to ngo ionization. In fact, the stabilization of the corresponding MO by 0.7 eV relative to 2-methoxytropone (Table 1) agrees well with the expected I shift due to hydrogen-bond formation. For instance, the corresponding shift on passing from 2-methoxycyclohexanone to 2-hydroxycyclohexanone is found to be 0.6 eV [ 201. Given this assignment for band 2, then ioniz- ations of 7r(5a”)-i and 7r(4a”)-l follow necessarily for bands 1 and 3, respectively (Table 1). The effect of hydrogen-bond formation is clearly discernible in Fig. 2.

This assignment is confirmed further by an analysis of the vibrational progressions observed in the spectra, even though experimental errors in the fine-structure intervals prevent a conclusive discussion. Band 1 shows well-resolved structures (Fig. l(b)) with two discernible progressions (? N 1400 and v” N 360cm-’ ) that can be related respectively to the IR Raman stretching vibrations xczc and xczo and to the skeletal deformation modes of tropolone [21]. The band 2 is structured with a vibrational pro- gression of ? N 1100 cm-’ which may be correlated with C-C IR Raman stretching [21]. This observation agrees well with the expected through- bond mixing of the nczo orbital with more-internal u orbit&. Further evidence in support of the proposed assignment of band 2 to n&o ioniz- ation is provided by the relative-intensity variations upon switching from He(I) to He(I1) radiation for tropolone and 2-methoxytropone. Some enhancement of band 2, relative to all other bands up to 12 eV, is observed. This intensity pattern, within the framework of the Gelius model of PE cross-sections [22a], could be related to an increased 0 2p/C 2p cross- section ratio for the He (II) wavelength [22b], even though it should be remembered that, owing to the close vicinity of the n& and z-l ionizations, vibronic mixing may be a determining factor for He(I) versus He(I1) intensity ratios.

We now discuss the electronic structure of the anion ligand tropolonate using the Tl(1) complex which is suitable for PE investigations. Although covalent interactions are relevant in metal-ligand bonding in Tl(1) com- plexes, the PE spectra prove to be a powerful tool in elucidating the electronic structure of the bonded anion ligand [3-5,231.

The PE spectrum of Tl(1) tropolonate (Fig. 4) shows, in the low-1 region (<12eV), three bands (labeled 1-3 in Fig. 4 and Table 2) having relative

159

TABLE 2 IONIZATION-ENERGY DATA (eV) AND ASSIGNMENTS FOR Tl(1) TROPOLONATE

1 (eV) Assignment

7.75 8.85 9.30 9.65

11.41 12.23 13.64 14.62 16.01

lrbl ring + n+ 01 w ss2)

na2 + n-

I

u region

19.79= Tl 5d 2D,,2 21.93a Tl Sd 2D,,2

*Taken from He(I1) spectrum.

intensities 2:1:3. X-ray diffraction studies [17, 181 have shown that in the metal-bonded tropolonate anion there are no significant differences in the lengths of the two C-O bonds, the anion having local CzV symmetry. The C-C bond length in the O-C-C-O fragment is slightly shorter than the value quoted for a single C-C bond. Therefore, this bond participates in the R system to only a small extent. As a consequence, the electronic structure of the O-C-C-O fragment can be described in the same way as for (x- dicarbonyl systems. This means that the “in-plane” nonbonding oxygen 2p lone pairs will give rise to a pair of symmetry combinations, generally labeled n+ and n-, whose degeneracy is removed by through-bond mixing with other skeletal MO’s of suitable symmetry [ 241. The final energy-ordering found in a large variety of o-dicarbonyls is e(n+) > e(n-) (I@-) > I@‘) under Koopman’s approximation [24]). A similar sequence is observed in the PE spectra of cyclobutene-1,2dione and of o-benzoquinone, where the cudicarbonyl fragment is part of an analogous system [25] *. As far as the R MO’s are concerned, no relevant modifications are expected on going from neutral tropolone to the anion form. Therefore, the MO sequence pictured in Fig. 5 can be anticipated for the tropolonate anion ligand. This general sequence is confirmed by a semiempirical quantum-mechanical calculation (CNDO) for Na(1) tropolonate (r(b, ) > IZ+ > n- > r(az )). The above scheme can be modified to fit the case of the Tl(1) complex simply by including the effect of the expected “covalent” interaction involving the II+

* A similar trend is also observed in the PE spectra of o-tropoquinone [26] I

160

h/ V’L 19 21 23 d

/ 7 9 11 13 ev

Fig. 4. He(I) and He(II) PE spectra of Tl(1) tropolonate.

(a1 in Czv symmetry) combination and the so-called thallium 6s2 “inert” lone pair, again of a1 symmetry. Such an interaction results in a couple of MO’s of al symmetry which will be an admixture of both n+ and Tl 6s orbitals. It is ‘difficult to anticipate which of the corresponding combin- ations will be dominated by metal character. The final MO sequence can account for the spectrum of the Tl(1) complex (Fig. 5). Therefore, band 1 certainly represents ionization of the x(bl ) MO. Its relative intensity, however, indicates that the band must include other ionization events, and, because of the mentioned n+-Tl 6s2 interaction, it becomes conceivable to locate also the ionization of the antibonding cl MO over band 1. The following narrower band 2 at 8.85 eV finds no counterpart in the spectrum of tropolone. However, comparison with the PE spectra of various Tl(1) complexes [ 3-51, and in particular with that of Tl(1) acetylacetonate [3], suggests that this band is the best candidate to account for ejection of electrons from the remaining a1 MO, mainly Tl 6s in character. Finally, band 3 must necessarily include both (n-)-i and r(a2 )-’ ionizations, because of the observed intensity ratio relative to the previous bands. The change of the relative band intensities when using He(I1) radiation provides

161

= bl “1 ------------- .’ 81

‘. _’ ‘a.

,I. .\ ,

/ .\

n+a, \ .’ ‘\ \ \\ 652

‘. *. ;,- TI

‘. .’

,- ‘.

’ *._, 81 ,/

,’

n- b2 b, -_-_-______ --________

= a2 a2

trop- TI trop TI+

CPV c2v

Fig. 5. Qualitative scheme for uppermost filled MO of Tl(1) tropolonate.

further confirmation of this assignment. The intensity of band 2 increases with respect to bands 1 and 3, as expected because of the increased Tl 6s/O 2p cross-section ratio at higher photoionizing frequency [ 27].*

Interestingly, the He(I1) PE spectrum of TI(1) tropolonate (Fig. 4) shows an intense doublet in the 19-22 eV region, necessarily absent in the He(I) spectrum. Reference to literature data indicates clearly that it must be related to the 2Ds,2 and 2D3/2 final states produced by removing one electron from the thallium 5d’* ground configuration. Unlike the case of the thallium halides 1233, the bands do not show evidence of fine structure due to @and-field effects, A similar observation has been made already for other “covalent” Tl(I) complexes and attributed to the almost electrostatic origin of ligand-field perturbation upon the “core-like” thallium 5d subshells [3, 23). The spin-averaged metal 5d I (20.65eV) is lower than the value reported for the neutral Tl atom (21.38 eV) but comparable to those for Tl(C!sHs ) (20.30 eV) and Tl (I) acetylacetonate [ 271. Such a “chemical shift” would lead to the counterintuitive conclusion that the thallium atom bears a negative charge. However, we believe that a more convincing explan- ation of the observed I trend is in terms of extra-atomic relaxation effects in the d 9 hole state, as proposed in the case of the Tl (C, HS ) complex [ 23 ] . In other words, enhanced stabilization of the strongly localized d9 hole state by the easily polarizable electron cloud of the ligand results in stabilization of the ion state ground state.

and, thus, in a smaller energy difference with respect to the

* A similar trend complexes.

has been observed in the PE spectra of various Tl(1) b-diketonate

162

ACKNOWLEDGMENT

The authors gratefully acknowledge Professor E. Heilbronner for helpful comments and valuable discussion of this paper.

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